1. If Federer withdraws before the draw is made (which is scheduled for 10:00 a.m. Friday) , everybody below him will move up by one spot in the seedings (and thus Rafa would get #4 seed among other things).
2. Nadalites do not need to worry. Even if Roger withdraws after the draw, Rafa will get the #4 seed as long as the tournament rebalances the draw. In the event Fed withdraws after the draw is made but before the order of play for the opening day is released (which is scheduled for 11:00 a.m. Saturday, remember that at RG the tourney starts on sunday as they play the 1st round spread over three days), the tournament organizers are required to rebalance the draw as per the following rule. Any vacancy created by the withdrawal of a seed prior to the release of the Order of Play for the first day of Main Draw shall be filled as follows: If the withdrawal is among seeds 1 to 4, the 5th seed is moved into then open position, the 5th position shall be filled by the 17th seed, and the 17th position shall be filled by the next highest ranked player eligible to be seeded. So, if that situation arises, Rafa will get #4 Seed, John Isner will get #5 Seed, Fog will get #17 seed and a lucky loser will take Fog's place.
3. If Fed withdraws after the OOP for opening day is released but before play commences on the opening day, it is upto the discretion of the tournament organizers whether they want to rebalance the draw or not. However, if they choose to rebalance the draw, they must follow the abovementioned procedure.
4. If they choose not to rebalance or if Fed withdraws after the first ball is struck, Fed's position will simply be given to a lucky loser.
p.s. Hopefully, Fed will not withdraw and we don't have to worry about all these rules.